Semiconductor devices are used for integrated circuits in a wide variety of electronic applications, such as computers, cellular mobile phones, radios, and televisions.
One particular embodiment of a semiconductor device is a magnetoresistive random access memory (MRAM) which uses spin electronics for storing information therein. Spin electronics combines traditional semiconductor technology and magnetism. Rather than using an electrical charge to indicate the presence of a binary logic “1” or logic “0”, the magnetic state of a storage cell is used to represent the binary information. In a common magnetoresistive random access memory, a magnetic storage cell is provided, usually comprising a first magnetic layer (also referred to as a reference layer) and a second magnetic layer (also referred to as a storage layer), the magnetic orientations thereof can be changed. The relative magnetic orientation of the two magnetic layers results in different ohmic resistances of the storage cell. By way of example, in case of a parallel magnetic orientation of the two magnetic layers, a low ohmic resistance results, whereas in case of an anti-parallel magnetic orientation of the first magnetic layer and the second magnetic layer, a higher ohmic resistance results. Thus, by aligning the magnetization of the storage layer parallel or anti-parallel with respect to a reference layer, the change of the ohmic resistance of the cell can be used to store the information in the MRAM cell.
One type of MRAM called thermal select MRAM has the high stability against thermal fluctuations and requires small switching fields for programming. With the reduction of the size of the MRAM cells below about 150 nm, thermal select MRAM cells are robust to thermal fluctuations, since the storage layer is pinned to an anti-ferromagnet, thereby stabilizing it against thermal fluctuations. The cell is programmed by heating it over the blocking temperature by means of a current (the temperature, at which the exchange coupling and therewith the pinning disappears) and by cooling it down in the presence of a counter field (so called thermal writing). This counter field re-magnetizes the storage layer, the new orientation of which is frozen during the cooling down process when becoming lower than the blocking temperature. Thermal select MRAM cells are therefore suitable to be used in high density MRAM devices.
When reading the information stored in a MRAM cell, due to the variations of the cell resistance on a chip caused by the manufacturing thereof, the information of the cell is usually read in such a way that its resistance is compared with the resistance of a reference cell. This would require additional reference cells and additional chip area for the reference cells, thus decreasing the density of MRAM devices.
In another approach, so-called self-reference may be used to read the stored information. As shown in FIG. 1, a typical MRAM cell 100 comprises a reference layer 110, a tunnel barrier layer 120 and a storage layer 130, wherein the information is written into the storage layer 130 by aligning the magnetization 132 of the storage layer 130 to be parallel or anti-parallel to the fixed magnetization 112 of the reference layer 110. Whereas in a self-reference MRAM cell, the information is written into the reference layer 110 or the reference layer system. The original storage layer 130 is used as a read layer or a self-referencing layer. In this case, the states, between which the read layer 130 switches, are always parallel or anti-parallel with respect to the magnetization 112 of the reference layer 110. The reading of the self-reference MRAM cell is carried out by a write operation and read operation which are repeated twice. In this case, the cell is switched twice and the corresponding resistance values are compared with each other to determine the information stored in the self-reference MRAM cell.
The possibility to use self-reference would be helpful for small cells, for example, the thermal select MRAM cells, since they would impose less requirements, e.g., to the TMR (tunneling magnetoresistance signal) signal and the resistance distribution. However, it would need more time and more power consumption due to the plurality of switching and reading.